The term "permissible" describes explosives that are cap-sensitive and relatively non-incendive so that they can be used in the underground mines having potentially flammable atmospheres, such as underground coal mines. The Mine, Safety and Health Administration of the United States Department of Labor has established detailed requirements for approval of permissible explosives for underground use. These requirements are published in 30 C.F.R. Part 15. These regulations, which are incorporated herein by reference, define permissible explosives in terms of stringent minimum performance requirements.
By "low density" is meant explosives having a bulk density of less than 1.0 g/cc, and preferably about 0.9 g/cc. The low density explosives of the present invention have lower detonation velocities and bulk energies than higher density couterparts. For example, prior art compositions generally have densities above 1.0 g/cc and detonation velocities of about 4,700 m/sec or higher; whereas, the present compositions have densities below 1.0 g/cc and velocities of about 4,200 m/sec or less. This is advantageous for blasting in coal mines where lumps rather than finer fragments generally are desired. The low velocity allows for a heaving rather than shattering action on the soft coal body. A lower detonation velocity also correlates generally with less incendivity which also is desirable for permissible blasting applications. Shock resistance is provided in the present invention by the use of relatively high strength glass or plastic hollow spheres. By "shock-resistant" is meant the ability to withstand shock wave desensitization that commonly is referred to as "dead pressing." The use of high strength hollow spheres to prevent dead pressing in slurry explosives is disclosed in U.S. Pat. No. 4,474,628. The hollow spheres for use in the present invention need to have a strength sufficient to withstand or resist the shock from a neighboring detonation, or in other words, to resist dead pressing. But high strength hollow spheres, by themselves, do not impart enough sensitization to the explosives of the present invention.
In order to achieve shock resistance and adequate sensitivity for permissible applications, it has been found necessary to use both high strength hollow spheres for shock resistance and chemically produced gas bubbles for sensitivity. If only high strength hollow spheres are used to reduce the density of the explosive and thereby increase its sensitivity, the sensitivity is not increased sufficiently to meet the permissibility requirements. Moreover, high strength hollow spheres are relatively expensive, particularly if used as the sole density reducing means. On the other hand, gas bubbles alone can achieve the required sensitivity levels, but they do not provide sufficient resistance to dead pressing or shock. Thus it has been found in the present invention that lowering the density to the required range by the combination of high strength hollow spheres and chemically produced gas bubbles provides the necessary shook resistance and detonation sensitivity, and also imparts a lower detonation velocity to the explosive.
Although the combination of gas bubbles and hollow spheres for density reduction in emulsion explosives has been previously suggested, for example see U.S. Pat. Nos. 4,594,118 and 4,474,628, the use of high strength hollow spheres in combination with chemical gassing to produce a low density, shock-resistant permissible emulsion explosive is not disclosed in the prior art.
Although most prior art compositions have densities greater than 1.05 g/cc, lower density ranges also have been disclosed generally in certain prior art patents, for example, U.S. Pat. Nos. 4,790,891; 4,737,207; 4,711,678; 4,594,118; 4,566,920; 4,547,234; 4,394,198; 4,383,873; 4,287,010; 4,149,917; 4,110,134; 3,642,547; 4,322,258; 4,216,040; and 4,141,767. Here again, none of these references disclose the combination of the present invention, and particularly not in a permissible composition.